In a digitally controlled printing system, for example an inkjet printing system, a receiver medium is directed through a series of components. The receiver medium can be a cut sheet or a continuous web. A transport system physically moves the receiver medium through the printing system. As the receiver medium moves through an inkjet printing system, liquid, (e.g., ink), is applied to the receiver medium by one or more printheads through a process commonly referred to a jetting of the liquid. The jetting of liquid onto the receiver medium introduces significant moisture content to the receiver medium, particularly when the system is used to print multiple colors on a receiver medium. Due to its moisture content, the receiver medium can undergo various physical changes, which can include change in electrical resistivity as well as changes in physical dimension and mechanical stiffness.
Cut sheet transport systems typically employ a holding force to hold the receiver medium to a transport mechanism, such as a belt or a drum. The holding force on the receiver medium can be applied using vacuum or electrostatic force, for example. Transport systems using electrostatic force can be advantageous because of their simplicity, and are frequently used in printing systems using dry printing processes, such as electrophotography. For example, U.S. Pat. No. 5,918,875 to Masley et al., entitled “Zero clearance handle,” describes a paper feeding system in which a copy sheet is charged so that the sheet is electrostatically secured (or “tacked”) to a photoreceptor belt. The sheet is then provided with an opposite charge so that it can be removed (or “detacked”) from the transport belt.
For printing systems where a liquid ink is applied to the receiver medium, the resulting change in electrical resistivity of the receiver medium can adversely impact the reliability of using electrostatic holding for the receiver medium. Moist paper can dissipate charge relatively quickly. Moreover, connected regions of liquid permit charge to move through them, potentially redistributing charge on the receiver medium. Evaporation of components of liquid ink can also result in ionization that neutralizes some of the charge on the receiver medium. Still, some inkjet systems can be compatible with electrostatic holding of the receiver medium. U.S. Patent Application Publication No. 2011/0109037 to Kunioka entitled “Sheet feeder and image forming apparatus incorporating same,” describes an ink-ejecting printer having recording heads mounted on a carriage over a transport belt. The transport belt electrostatically holds a receiver sheet and transports it to a position facing the recording heads. In a carriage printer, the printheads are moved by the carriage to print a swath of an image and advance the print medium between swaths in order to form the image swath by swath. In this application, the transport belt is able to provide sufficient electrostatic holding force for the portion of the cut sheet receiver medium that has not yet been printed. After the entire image is printed, the sheet is transported only a short way to an output tray.
High volume cut sheet printing systems typically print one color of an entire line of the image essentially all at once, for example using a page-width printhead or other page-width printing processes in a printing station for that color. The cut sheet is advanced past the printing station as page-width lines of the same color are printed sequentially. To print all colors (typically requiring at least cyan, magenta, yellow and black), the receiver medium is moved past a sequence of printing stations, one for each color. Dryers are typically provided between printing stations for evaporating volatile components of ink from the receiver medium, thereby increasing ionization due to evaporation. This can weaken the electrostatic holding force such that holding of the receiver medium is no longer reliable. If the receiver medium is no longer tightly held to the belt, registration of the colors printed by successive printing stations can be lost, thereby degrading print quality. Loosened receiver medium can also cause paper jams.
There remains a need for an electrostatic holding system that provides a reliable electrostatic holding force for the transport of sheets of receiver media through a cut sheet printing system. In addition, what is needed is an electrostatic holding system that accommodates dimensional changes in receiver medium that has absorbed liquid during the printing process.